1. Field of the Invention
The present invention relates to coating compositions. In particular, the present invention relates to solvent-containing coating compositions containing at least one hydroxyl-containing saturated copolyester, a phenol-formaldehyde resin, a benzoguanamine-formaldehyde resin and/or a blocked polyisocyanate, catalysts, solvents and, if appropriate, aids and additives.
2. Discussion of the Background
Coating agents are used for the external and internal lacquering of metal packaging articles. The coating agents are free of polyvinyl chloride (PVC), bisphenol A (BPA) and also bisphenol A diglycidyl ether (BADGE), bisphenol F diglycidyl ether (BFDGE), novolac glycidyl ether (NOGE) and derivatives thereof. The coating agents are suitable for roller coating of metal sheets or coils, in particular for highly deformed metal packages, having excellent adhesion and stability to sterilization, especially in acid media, such as acetic acid and lactic acid.
Metal packaging articles for foods and drinks, such as cans, tubes and closures, are customarily provided with an organic coating (see “Canmaking, The Technology of Metal Protection and Decoration” T. A. Turner, Blackie Academic & Professional, London, 1998, page 82; “Polymeric Materials Science and Engineering”, Vol. 65, fall meeting 1991 New York, pages 277–278), in order firstly to protect the metal from corrosion, to ensure further processing, and secondly to protect the filled contents, so that the food constituents are retained. Also, the color, texture and flavor of the filled contents must not be changed over the period of the storage life of the metal packaging articles. Therefore, transfers of metal into the filled contents are also to be prevented by the organic coating. Furthermore, the filled contents must not be affected or changed by the lacquer layer itself or parts of the lacquer layer which have been leached out (“Verpackung von Lebensmitteln” [Packaging of foods] Rudolf Heiss, Springer Verlag, Berlin 1980, page 234 ff).
The coating agents are applied in the roller coating process either continuously on coil lines or batchwise on sheet coating lines to thin metals such as aluminum, tinplate or chromed steel (TFS=tin free steel) and then reacted at high temperatures.
The coated metals thus produced are then shaped to form the desired metal packaging articles by processes such as, for example, deep-drawing, stamping, creasing and flanging. This machining requires very high flexibility and excellent adhesion of the coating agents used. They must not experience any change in the protective function due to the reshaping processes and must then have perfect adhesion and intact surfaces.
Many metal packaging articles, after filling with the foods, are subjected to processes for preservation. These are, for example, pasteurization or sterilization; after these thermal processes also, the coating agents must not have any change with respect to protective function, adhesion and surfaces.
Currently, coating agents especially for direct food contact, based on epoxy-phenol lacquers, are used. These coating agents are called “gold lacquers” (see “Ullmann's Encyclopedia of Industrial Chemistry”, Vol. A 18, VCH Verlagsgesellschaft, Weinheim 1991, page 527). These lacquers exhibit good adhesion and are particularly highly suitable for aggressive filled contents, in particular for acid filled contents. If particularly high demands are made of the flexibility of the coating agents, frequently PVC dispersions are used. These PVC dispersions are termed organosols, and are distinguished by particularly good adhesion and good deep-drawing properties.
However, not only the epoxy-phenol lacquers but also the organosols contain BADGE (bisphenol A diglycidyl ether(2,2′-bis(4-hydroxyphenyl)propane bis(2,3-epoxypropyl)ether) and/or BPA [bisphenol A=(2,2′-bis(4-hydroxyphenyl)propane)] are used. In addition, BFDGE and NOGE are also used. The PVC organosols additionally contain PVC.
Currently, the food packaging industry and consumer groups are demanding coated metal packaging articles in which the lacquers are firstly completely free from epoxy compounds (BADGE, BFDGE, NOGE and derivatives thereof), bisphenol A and PVC—and derivatives thereof, and secondly have comparable properties to the organic coatings previously used.
Bisphenol A is under suspicion of developing hormone-like activities in the human organism, hence the demand for its disuse (“Bisphenol A, A Known Endocrine Disruptor” WWF European Toxics Programme Report, April 2000, page 11). The use of BADGE and BFDGE with the respective limiting values is controlled in Europe by Commission Directive 2002/16/EC (see also Council Directive 89/109/EEC, Directive 90/128/EEC), and the use of NOGE has been banned since the beginning of 2003 for coatings having direct food contact.
Polyesters in combination with crosslinking partners such as melamine resins, benzoguanamine resins or blocked polyisocyanates are generally suitable for the external lacquering of metal packaging articles. They are flexible and may be sterilized in water without problems and are thus part of the prior art (Protective Coatings, Clive H. Hare, Technology Publishing Company, Pittsburgh, USA, 1994, page 149; Tinplate and Modern Canmaking Technology, E. Morgan, Pergamon Press, Oxford, page 195 ff; Can making, The technology of metal protection and decoration, T. A. Turner, Blackie Academie Professional, London 1998, page 40 ff).
Titanium-dioxide-pigmented polyester-benzoguanamine lacquers are used for the internal coating of cans, in which case solely high-molecular-weight polyesters (Mn>12 000) offer sufficient sterilization stability in acidic filled contents.
U.S. Pat. No. 6,472,480 describes polyesters in combination with phenoxy-group-containing resins or amino-group-containing resins. All polyesters suitable for the application must contain naphthalenedicarboxylic acid and also have branched structures by using trifunctional monomers such as TMA or TMP. The suitability of this coating composition, especially for sterilizations in acid media, is questionable.
DE 199 12 794 (Grace) describes BADGE-free can coatings, but includes bisphenol A as a component (see polyester 5, table V, comparative example B).
Kansai Paint Co. describes polyester phenol coatings which likewise contain bisphenol A. (JP 200220141, JP 200113147, JP 11315251).
GB 349464 (ICI) describes the use of “glyptal resins” (polyesters from glycerol and phthalic anhydride) in combination with phenol-formaldehyde resoles and urea-formaldehyde resins in aqueous emulsions. These coating compositions are not suitable for the application “metal packaging lacquers”.
The polyester-phenoplast coatings described in GB 1119091 (Schenectady) are intended for electrical insulation lacquers (wire enamels).
The heat-curable coating composition described in DE 22 28 288 (Stolllack) describes phenol-alkyd resin lacquers which are intended to avoid the phenomenon of sulfurizing. The proportion of the phenol-formaldehyde resin varies between at least 50% to 95%. If the proportion of phenol falls to 30%, the resultant lacquer is sticky. The binder used is of the alkyd type, that is to say an oil-containing polyester, and is used at 10 to 30%. The adhesion of this coating composition after mechanical shaping and sterilization was not described and is still questionable.
DE 40 10 167 (BASF) describes a process for the interior coating of metal sheet packaging. The coating compositions described are a carboxyl-group-containing polyester and a phenol resin. Disadvantages of this invention are the use of one or more epoxy resins to improve adhesion, and also the use of thermoplastic resins.
WO 95/23198 (DSM) describes the use of a hydroxyl-group-containing polyester for the interior coating of cans. Disadvantages of these described polyesters are that they must have a glass transition temperature of at least 40° C., and, inter alia, epoxy resins must be used as crosslinking partners.
Polyester-phenol-formaldehyde combinations exhibit after shaping what are termed microcracks, for example at the corners of cans or at wedge-bend samples. These microcracks are faults in the coating and are therefore not suitable for use as interior protective lacquer of metal packages, since here the protective function of the coating is permanently disturbed (See example 8, table IX, coating agent C).
JP 2002302643A describes the use of branched copolyesters for interior coating of cans. The crosslinker used is not described. Mention is made of generic terms such as aminoplast resins, phenolic resins, melamine resins, polyfunctional isocyanates, blocked isocyanates and polyfunctional aziridine resins which are used as crosslinkers of polyesters. This merely represents the general state of knowledge of the crosslinking reaction of polyesters (see: Lackharze [lacquer resins]; D. Stoye, W. Freitag; page 47; Carl Hanser Verlag, Munich 1996).
None of the patents describe the use of saturated hydroxyl-containing copolyesters in combination with phenol-formaldehyde resins, benzoguanamine resins and/or blocked polyisocyanates.